Chapter 7

Starts like this:

1 Since we have these promises, beloved, let us cleanse ourselves from every defilement of body and spirit, bringing holiness to completion in the fear of God. [2Co 7:1 ESV]

But that seems to me to be more the last verse of 6 than a new chapter.  The promises referred to are those in 16-18.  7:1 is the concluding verse of chapter 16.

In 2-4 Paul tells them of his own emotional feelings for them.  His pride in them.  He is, I think, overcome that they have at last thrown off their rebellion and turned back as much as they can to the original teachings of Paul.  His emotions are overwhelming him that the church there is not going to be in vain, or lost to the deceit of the false apostles and of Satan who sent them.

vss 5-9 confirm that I'm on track with what I said about 2-4.  Paul says his severe letter caused them "Godly grief", a good thing in his eyes, with many good long term results, as in these verses:

10 For godly grief produces a repentance that leads to salvation without regret, whereas worldly grief produces death. 11 For see what earnestness this godly grief has produced in you, but also what eagerness to clear yourselves, what indignation, what fear, what longing, what zeal, what punishment! At every point you have proved yourselves innocent in the matter. [2Co 7:10-11 ESV]

2021-2, "the matter" is singular.  That severe letter must have addressed one specific heresy perhaps.  Acceptance maybe of just one highly immoral person.  We don't know for sure.  But here is a clue that it was "a matter", not a broad departure of the whole church.  In the next verse, 12, we see that something was going on with two people.  Just nearly impossible to speculate even.  We could be so far off.